The present invention generally relates to magnetic memory devices and more particularly to a magnetic random access memory capable of writing information with a reduced write-current.
A magnetic random access memory (MRAM) is a magnetic memory device that stores information in the form of magnetization in a ferromagnetic layer constituting a part of a magneto-resistive sensor. The information thus written into the ferromagnetic layer is read out by detecting a magneto-resistance of the magneto-resistive sensor.
A MRAM is characterized by high operational speed comparable to that of a static random access memory (SRAM) and a simple construction suitable of forming a high-density integrated circuit. In addition, a MRAM has an advantageous feature of immunity to soft-errors caused by penetration of charged particles such as cosmic ray.
With the progress in the art of GMR (giant magneto-resistive) sensors, particularly with the progress of a TMR (tunneling magneto-resistive) sensor, application of MRAM is expanding rapidly in various fields of electronics including computers and telecommunication.
FIGS. 1A-1C are diagrams showing the principle of a MRAM according to a related art (S. S. P. Parkin, et al., J. Appl. Phys. vol.85, pp.5828, 1999).
Referring to FIG. 1A showing a single MRAM cell, the MRAM cell includes a word line 56 extending in a first direction and a bit line 57 extending in a second, different direction, and a TMR sensor is interposed between the word line 56 and the bit line 57 in correspondence to an intersection part where the word line 56 and the bit line 57 cross with each other.
More specifically, the TMR sensor includes a ferromagnetic free layer 52 provided on the word line 56 via an intervening foundation layer 51, and a tunneling barrier layer 53 of AlOx and a ferromagnetic pinned layer 54 are formed consecutively on the foregoing ferromagnetic free layer 52. Further, there is provided an anti-ferromagnetic pinning layer 55 between the ferromagnetic pinned layer 54 and the bit line 57, wherein the anti-ferromagnetic pinning layer 55 causes a pinning of magnetization in the pinned layer 54 in a predetermined direction represented in an arrow.
When writing information, a word line current is caused to flow through the word line 56 in the direction represented by an arrow in FIG. 1B together with a bit line current, which is caused to flow through the bit line 57 in the direction represented by another arrow in FIG. 1B. Thereby, the bit line current and the word line current induce respective magnetic fields 57 and 58 as represented in FIG. 1B, and a synthetic magnetic is field formed as a result of a sum of the magnetic fields 57 and 58. The synthetic magnetic field thus formed induces a magnetization in the ferromagnetic free layer 25 as represented by an arrow in FIG. 1B. In the state of FIG. 1B, it can be seen that the magnetization in the free layer 52 and the magnetization in the pinned layer 53 are in an anti-parallel relationship. The state of FIG. 1B represents a data bit xe2x80x9c0.xe2x80x9d
Alternatively, the ferromagnetic free layer 52 may be magnetized in a parallel relationship as represented in FIG. 1C. The state of FIG. 1C may be designated as a data bit xe2x80x9c1.xe2x80x9d The information thus written into the MRAM cell is held stably even when the electric power to the MRAM cell is turned off. In other words, MRAMs function as a non-volatile memory.
In the state of FIG. 1B, there is formed a large magneto-resistance between the ferromagnetic free layer 52 and the ferromagnetic pinned layer 54 in correspondence to the anti-parallel relationship of the magnetization in the layers 52 and 54, while in the state of FIG. 1C, there is formed a small magneto-resistance in correspondence to the parallel relationship of the magnetization. Thus, reading of information can be achieved by detecting a magneto-resistance of the TMR sensor formed between the word line 56 and the bit line 57. Such reading of information is non-destructive reading.
In a TMR sensor, it should be noted that a ratio of tunneling resistance change or TMR ratio xcex94R is defined as
xcex94R=2P1xc3x97P2/(1xe2x88x92P1xc3x97P2) xe2x80x83xe2x80x83Eq.(1) 
wherein P1 represents a spin polarization of the pinned layer 54 while P2 represents a spin polarization of the free layer 52.
Thus, a TMR ratio xcex94R of about 50% is achieved when a NiFe alloy, having a spin polarization of about 45%, is used for the free layer 52 and also for the pinned layer 54:                               Δ          ⁢                      xe2x80x83                    ⁢                      R            max                          =                  2          xc3x97          0.45          xc3x97          0.45          ⁢                      (                          1              -                              0.45                xc3x97                0.45                                      )                                                  =                              0.405            /            0.7975                    ≈                      0.508            .                              
When writing information into the magnetic random access memory of FIG. 1A, it is necessary to apply an external magnetic field (Hx, Hy) such that the external magnetic field exceeds a critical value given by a relationship of
(Hx/Hx0)⅔+(Hy/Hy0)⅔=1 xe2x80x83xe2x80x83Eq.(2) 
wherein Hx0 represents a coercive force in the direction of easy axis of magnetization while Hy0 represents a coercive force in the direction of hard axis of magnetization.
FIG. 2 shows an asteroid curve corresponding to the relationship of Eq.(2) above, wherein there occurs an inversion or reversal of magnetization in the free layer 52 when the external magnetic field (Hx, Hy), caused by the magnetic fields 57 and 58, has exceeded the closed region defined by the asteroid curve.
From FIG. 2, it will be understood that the word line current and the bit line current for creating the inverting magnetic field (Hx, Hy) should have the same magnitude in order to minimize the magnitude of the word line current and further the bit line current.
On the other hand, such a MRAM has a problem in that the magnitude of the external magnetic field needed for inverting the magnetization of the magnetic free layer 52 increases with increasing degree of device miniaturization. It should be noted that the relative ratio of the thickness of the ferromagnetic free layer 52 to the lateral size thereof, and hence the structural anisotropy of the ferromagnetic free layer 52 in which the information is stored, increases with increasing degree of device miniaturization.
In the case a ferromagnetic free layer 52 is formed of a NiFe alloy, the external magnetic field needed for causing an inversion of magnetization in the free layer 52 reaches as much as 50-100 Oe when the MRAM is formed according to the 0.1 xcexcm design rule, in which the bit lines 57 are formed with a 0.1 xcexcm line-and-space pitch. In order to create the foregoing magnetic field by the electric current flowing through the bit line 57 and the word line 52, a current density of as much as 3-5xc3x97107A/cm2 is needed, provided that the word line 52 and the bit line 57 are offset from a thickness center of the MRAM cell by a distance of 0.1 xcexcm. With such a large electric current density, even a Cu conductor pattern, which is thought immune to electro-migration up to the current density of 106A/cm2, would cause electro-migration. This means that miniaturization beyond the 0.1 xcexcm design rule is not possible in the MRAM of FIG. 1A.
Of course, the word line current and the bit line current needed in a MRAM for writing information can be reduced by adding impurity elements to the ferromagnetic free layer 52 such that the coercive force thereof is reduced or the saturation magnetization is reduced. However, such an approach is thought undesirable as it would degrade the performance of the magnetic material and hence the TMR ratio xcex94R of the TMR sensor and the S/N ratio of the MRAM.
Generally, the TMR ratio xcex94R of a TMR sensor can be increased when the value of the spin polarization P1 or P2 is increased. For example, the TMR ratio xcex94R becomes theoretically infinite when magnetic materials having P1=1 and P2=1 are used for the ferromagnetic free layer 52 and the ferromagnetic pinned layer 54. However, the use of such materials increases the coercive force also, and writing of information with small word line current and bit line current becomes difficult.
Accordingly, it is a general object of the present invention to provide a novel and useful magnetic random access memory and a fabrication process thereof wherein the foregoing problems are eliminated.
Another object of the present invention is to provide a magnetic random access memory suitable for device miniaturization.
Another object of the present invention is to provide a magnetic random access memory capable of writing information with reduced electric current.
Another object of the present invention is to provide a magnetic random access memory, comprising:
a word line extending in a first direction;
a bit line extending in a second, different direction; and
a tunneling magneto-resistive sensor provided between said word line and said bit line in a part where said word line and said bit line cross with each other,
said tunneling magneto-resistive sensor comprising:
a first ferromagnetic layer having a fixed magnetization;
a second ferromagnetic layer having a variable magnetization; and
a tunneling insulation film interposed between said first ferromagnetic layer and said second ferromagnetic layer,
said first ferromagnetic layer comprising a ferromagnetic material having a spin polarization of 0.9 or higher.
According to the present invention, the TMR ratio of the tunneling magneto-resistance sensor is increased as a result of use of the magnetic material having the spin polarization of 0.9 or higher while simultaneously enabling writing of information with a small electric current.
Another object of the present invention is to provide a magnetic random access memory, comprising:
a word line extending in a first direction;
a bit line extending in a second, different direction;
a tunneling magneto-resistive sensor provided between said word line and said bit line in a part where said word line and said bit line cross with each other, said tunneling magneto-resistive sensor comprising: a first ferromagnetic layer having a fixed magnetization; a second ferromagnetic layer having a variable magnetization; and a tunneling insulation film interposed between said first ferromagnetic layer and said second ferromagnetic layer; and
a magnetic source applying an offset magnetic field to said tunneling magneto-resistive sensor.
According to the present invention, the magnitude of the magnetic field needed for causing inversion of magnetization in the magnetic free layer is reduced substantially by applying an offset magnetic field to the magnetic sensor.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.